This invention relates to adiabatic surface combustion with excess air to yield combustion products or flue gas with a limited content of atmospheric pollutants. More particularly, radiant, surface combustion burners are operated with excess air under conditions that ensure a remarkably low content of nitrogen oxides (NO.sub.x), carbon monoxide (CO) and unburned hydrocarbons (UHC) in the resulting flue gas.
In recent years there has been a considerable expansion in the use of flameless surface combustion burners that yield a substantial portion of the combustion heat as infrared radiation. Hence, these burners are often referred to as radiant flameless burners.
There are several types of surface combustion burners that have been proposed. U.S. Pat. No. 3,833,338 to Badrock shows a burner having a layer of deformable, porous ceramic fibre material through which the air and gaseous fuel permeate to be ignited on the exposed surface of the layer. U.S. Pat. No. 4,252,520 to Bratko discloses an infrared burner that is formed of a pervious self-supporting matrix being a fibrous molded member of alumina silica composition with a substantial chromic oxide content. A radiant surface combustor that has been finding widening uses is basically described in U.S. Pat. No. 3,179,156 to Weiss et al as having a fibrous porous wall or layer deposited upon a screen. U.S. Pat. No. 3,383,159 to Smith teaches adding a small amount of aluminum powder to the fiber burner of Weiss et al. U.S. Pat. No. 4,746,287 to Lanutti offers the improvement of aluminum alloy powders in porous fiber burners.
U.S. Pat. No. 4,597,734 to McCausland et al discloses another type of surface combustion radiant burner wherein the porous wall or layer on which surface combustion is conducted is formed from randomly laid fibers of an iron, chromium and aluminum alloy having the property of forming an alumina layer on the metal surface upon heating in the presence of oxygen. Still another type of surface combustion burner is provided by using a reticulated ceramic structure, as taught by U.S. Pat. No. 4,568,595 to Morris, for the porous layer on which surface combustion is carried out. Such a porous surface combustor is more simply called a ceramic foam burner.
The various types of surface combustion burners have in common the structural feature of a wall or layer having a multiplicity of fine foramina or pores through which the mixture of gaseous fuel and air must pass before being ignited at the exterior face of that layer. Similarly, these various burners have in common the operational feature that the fuel-air mixture burns at the exterior face of the porous layer without discernible flame and that exterior face becomes incandescent. Hence, these burners are often referred to as flameless and/or radiant surface combustion burners.
For the purposes of this invention, the various types of surface combustion burners will be included in the simple generic term, porous surface combustor. Compared to other burners that produce flames, porous surface combustors have the distinct advantage of yielding comparatively small amounts of the atmospheric pollutants: NO.sub.x, CO and UHC. However, these small amounts of pollutants are achieved only if the quantity of excess air used in the combustion mixture is not large. U.S. Pat. No. 4,519,770 shows graphically in FIG. 6 that CO remains below 30 parts per million (ppm) in the flue gas so long as the air fed to the porous surface combustor does not exceed 50% in excess of the stoichiometric requirement. Similarly, UHC remains at about 2 ppm over the same limited range of excess air. At excess air quantities greater than 50%, the quantities of CO and UHC increase tremendously. Fortunately, NO.sub.x is generated in very small amounts at all levels of excess air; in fact, the suppression of NO.sub.x emission is an important characteristic of porous surface combustors that is responsible for the growing uses of these burners.
The use of a low amount of excess air is often desirable and advantageous. For example, where combustion heat is utilized indirectly to heat a fluid flowing through a metal tube as in the generation of steam, minimizing excess air minimizes the amount of heat lost with the vented flue gas. However, there are many cases where a large amount of excess air is preferred; for instance, where combustion heat is used directly by passing the hot products of combustion or flue gas in contact with a material such as rice to effect dehydration or a freshly coated paper to dry the coating. High excess air is also favored where the hot flue gas is discharged into a space such as a hog barn or a greenhouse to convey heat directly thereto. An environmentally important use of high excess combustion air is in the combustive clean-up of vitiated air resulting from some industrial operations like spray-painting because in such case for the sake of economy the object is to add the least amount of fuel that will achieve the combustive destruction of the pollutants in the vitiated air.
Accordingly, a principal object of this invention is to make it possible to operate porous surface combustors with more than 50% excess air while suppressing the formation of NO.sub.x, CO and UHC.
Another important object is to provide simple means for attaining the desired results.
Other features and advantages of the invention will be apparent from the description which follows: